1 . Field of the Invention
The invention concerns an antenna amplifier, in particular for a magnetic resonance antenna.
2 . Description of the Prior Art
In magnetic resonance technology, local surface coils (known as “loop antennas”) or array arrangements assembled from local coils are used for acquisition of image information of the examination subject. Such surface coils or antennas include the actual antenna or receiver component (resonator), the signals from which are supplied to an antenna amplifier that is disposed relatively close to the reception component. There the (possibly noise-afflicted) signal is amplified and conducted from the MR apparatus via a coaxial line for further processing. Sheath currents or the sheath waves as a consequence thereof that are created in the region of the coaxial line cause problems. These generate E-fields and B-fields that form between the “transparent” coaxial line and the surroundings. These sheath wave currents, although very weak in terms of their magnitude, influence the actual usable signal and can alter this to the point of unusability. More or less strongly pronounced positive and negative feedbacks that alter the originally completely smooth broadband frequency response in an unforeseeable manner are created in regions due to the phase changing with respect to the frequency. Given a matching phase, the arrangement (formed of the antenna and preamplifier) can even oscillate.
To suppress the influence of sheath wave currents, it is known to integrate a sheath wave barrier before the antenna amplifier, thus in the feed line from the antenna part to the amplifier. However, this worsens the noise ratio due to the damping effect of the inserted element, meaning that the noise component is increased, which is disadvantageous for the resolution of the useful signal. Another known solution is the insertion of one or more sheath wave barriers in the coaxial cable exiting from the antenna amplifier and traveling out of the apparatus. This approach, however, leads to sheath wave problems when it is desired to directly connect the ground electrodes of two amplifier outputs, as is desirable, for example, in circular polarized systems. At high frequencies in which a magnetic resonance apparatus is operated with a high basic field strength of 1.5 T and more, sheath wave problems also already occur due to the cable section exiting from the antenna amplifier up to the first sheath wave barrier.